The present invention relates to circuitry which precludes the energizing of a load from a three-wire, dual voltage, power distribution system whenever an open circuit exists in the system or correct connections have not been made thereto. Three-wire, dual voltage, AC power distribution systems are widely utilized for supplying electrical service to homes and industry. In such systems, an equal voltage is available between either of two hot lines and a neutral line, however, between the two hot lines the voltage magnitude is double. Therefore, conditions hazardous to equipment can occur when distribution boxes are incorrectly connected to such a system or when an open circuit develops in a line thereof. Such hazardous conditions are frequently encountered at construction sites where temporary power installations are utilized in which portable distribution boxes are randomly disposed and interconnected by electrical cables.
Circuitry is known by which power transmission through distribution boxes is precluded when improper connections have been made to the power distribution system or when an open circuit exists in a line thereof. However, all such circuitry is either expensive to incorporate or of limited adaptability to other desirable circuit refinements. One such circuit refinement is that after the distribution box has been energized from the system, power will continue to be transmitted therethrough from either hot line when an open circuit occurs in the other hot line. In some applications, this circuit refinement must be further sophisticated in that transmission of power is only continued if particular equipment, such as a DC power supply, is connected to the hot line having no open circuit therein. Another highly desirable refinement is to provide ground fault protection for the power transmitted through the distribution box.